Engines may be operated with boosted aircharge provided via a turbocharger wherein an intake compressor is driven by an exhaust turbine. However, placing a turbine in an exhaust system can increase engine cold-start emissions due to the turbine acting as a heat sink. In particular, engine exhaust heat during the engine cold-start may be absorbed at the turbine, lowering the amount of exhaust heat that is received at a downstream exhaust after-treatment device. As such, this delays attainment of light-off temperature at an exhaust catalyst, such as a diesel oxidation catalyst (DOC) or a selective catalytic reduction (SCR) catalyst. In addition, regeneration of a particulate filter (such as a DPF) may be delayed. Placing the turbine downstream of the exhaust aftertreatment devices may result in turbo lag during vehicle acceleration.
Accordingly, various approaches have been developed for routing exhaust through different exhaust system components based on temperature requirements. One example approach, shown by Andrews in U.S. Pat. No. 8,234,865 involves routing exhaust towards an exhaust tailpipe via a passage that bypasses the exhaust turbine during cold-start conditions. A passive, thermatically-operated valve is used to regulate the flow of exhaust through the passage, the valve opening during low-temperature conditions (such as during the cold-start). By circumventing the turbine, exhaust heat may be directly delivered to the exhaust catalyst.
However, the inventors herein have recognized potential issues with such a system. As one example, the temperature of exhaust reaching each exhaust system component (including the turbine, the DOC, the DPF, and the SCR catalyst) cannot be regulated. Further, it may be difficult to provide an exhaust flow that meets the conflicting heat requirements of different exhaust components. For example, after catalyst light-off, the temperature of unobstructed exhaust reaching the DOC may be higher than desired. In particular, owing to a coating on the DOC surface, the catalyst may have higher conversion efficiencies at lower exhaust temperatures. As a result, the higher than desired temperature of exhaust reaching the catalyst may result in reduced catalyst functionality. Also, a lower than desired temperature of exhaust reaching the DPF during DPF regeneration may result in incomplete regeneration. Similarly, a lower than desired temperature of exhaust reaching the SCR catalyst during a purge event may reduce the efficiency of NOx conversion. Further, locating a DOC, DPF, and/or SCR catalyst upstream of the exhaust turbine may result in turbo lag and boost pressure loss during acceleration and higher engine load conditions.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. In one example, the issues described above may be addressed by a method for an engine comprising: adjusting a plurality of valves coupled to each of a first, second, third, and fourth sub-branch of a branched exhaust system, each sub-branch arranged parallel to a main exhaust passage and housing a distinct exhaust component; and varying an order of exhaust flow through the distinct exhaust components based on exhaust temperature constraint. In this way, exhaust heat may be delivered to each exhaust system component based on the engine operating conditions and the desired operating temperature of the respective component.
In one example, a turbocharged engine system may be configured with a branched exhaust assembly wherein the exhaust passage is divided into at least three separate branches, each creating a distinct flow path. The main exhaust passage may constitute the central (main) branch, while a first peripheral branch may be further divided into three sub-branches and a second peripheral branch may also be divided into three sub-branches. The sub-branches and branches may be interconnected to each other via valves such that an order of exhaust flow along each of the flow paths can be adjusted via adjustments to a position of the valves. Distinct exhaust components may be coupled to the distinct sub-branches of the branched exhaust assembly. For example, an exhaust turbine of the turbocharger may be coupled to a first sub-branch of the first peripheral branch, a diesel oxidation catalyst (DOC) may be coupled to a second sub-branch of the first peripheral branch, a diesel particulate filter (DPF) may be coupled to a third sub-branch of the second peripheral branch, and a selective catalytic reduction (SCR) device may be coupled to a fourth sub-branch of the second peripheral branch of the exhaust assembly. One or more urea injectors may be coupled to the fourth sub-branch upstream of the SCR catalyst for injection of a desired amount of urea during NOx purging. Based on engine operating conditions, valve positions of the branched exhaust assembly may be adjusted to operate the exhaust system in one of a plurality of modes with exhaust routed in a distinct flowpath providing desired heat to each component of the system.
As an example, during cold start conditions, valve positions may be adjusted so that exhaust may bypass the turbine and first flow through the DOC in order to expedite attainment of catalyst light-off temperature. Thereafter, based on DPF and SCR loading, the exhaust may either flow through the DPF and then through the SCR or vice versa. As another example, during vehicle acceleration, in order to reduce turbo lag, valve positions may be adjusted so that exhaust may be routed first through the turbine and then through the remaining exhaust components before being released to the atmosphere. An order of exhaust flow through the DOC, DPF, and SCR catalyst may be determined based on the relative heat requirement of each component. For example, exhaust flow may be directed first through the DPF when DPF regeneration conditions are met, enabling a larger portion of the exhaust heat to be transferred to the DPF, while exhaust flow may be directed first through the SCR catalyst when SCR regeneration conditions are met, enabling the larger portion of the exhaust heat to be transferred to the SCR catalyst. Further still, during high engine load conditions when boost is required, the valves may be adjusted such that exhaust may be simultaneously routed to the tailpipe through two separate flow paths. For example, a first portion of exhaust may flow first through the turbine, bypassing the DOC, and then through the DPF and SCR while a second (remaining) portion of the exhaust may flow first through the turbine, bypassing the DOC, and then through the DPF and SCR before exiting via the tailpipe. The portion of the exhaust routed through the DOC relative to the portion routed through the turbine may be adjusted based on the boost demand.
In this way, by routing exhaust through different flow paths of a branched exhaust assembly, it is possible to expedite attainment of catalyst light-off temperature while assisting DPF regeneration and/or SCR purge during cold-start conditions, and without compromising boosted engine performance. Specifically, exhaust can be flowed through various exhaust components with an order of exhaust flow through the components adjusted based on their individual heat requirements. By routing exhaust via multiple flow paths in the exhaust assembly, it is possible to partially bypass the turbine with reduced reliance on a dedicated waste gate. The technical effect of regulating an order of exhaust flow through the exhaust components housed in distinct sub-branches of a branched exhaust assembly is that exhaust heat can be directed to a specific component first, as required based on engine operating conditions, irrespective of the positioning order of the exhaust components relative to each other in the exhaust assembly. In addition, the heating requirement of different exhaust components may be met even if they are conflicting. Overall, by changing an order of exhaust flow through exhaust components, engine efficiency, emissions quality, and fuel efficiency may be improved in a boosted engine system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.